It never was particularly safe, however in the bad old days, fewer people stole dogs to sell on, use for breeding or to ransom back to their owners, fewer people stole dogs to torture and kill.
More people told their kids not to approach other peoples dogs, and dogs that went out of their way to nip/bite were euthanised without fuss.
These days people do some horrific things, and expect dogs to tolerate any and all interactions, up to and including violence.
@Twinkleandstressed Please do not put your dog at risk again. Your dog felt threatened and being tied up will have made that much much worse. Using a muzzle to allow you to put them in that situation again may make you feel safe and it might (but not a guarantee by any means) make others feel safe, but it will make your dog feel even less safe. Surely your dog here is your priority?
Keep in mind that if your dog is tied up but you are not there, and your dog puts someone at reasonable fear of injury, you have committed the crime of having your dog dangerously out of control in a public place. Even though the dog is tied, if you're not there, right there, next to them, then you cannot possibly argue your dog is under control.
I would recommend muzzle training your dog for walks that involve busy pavements or places where you cannot see what is coming from a decent distance - avoid going to busy places - do not tie your dog up and leave them anywhere - do hire a behaviourist/trainer to work with your dog to get them feeling more relaxed and safe.
Doing all of this ASAP means if you get a knock on the door from the police later on, you can show you are a pro-active owner, already taking steps to prevent this situation occurring again. It may seem like over-egging the cake somewhat, but doing anything you can to visibly prove you're taking it seriously can make a huge difference to the outcome.
As it stands your dog has caused injury - the fact the injury wasn't a bite isn't relevant, the childs parents have every right to take legal action.